Which of the following is not an accessory structure of the respiratory system?
a. oral cavity
b. trachea
c. rib cage
d. diaphragm

The Trachea

Explanation:

The oral cavity aids in the transport of air when may be obstruction in the nasal cavity.

The Rib cage aids in the movements of respiratory inspiration and expiration  through the intercostal muscles and other estructures.

The Diaphragm aids in the regulation of the respiratory movements and the input and output of air into the lungs.

The Trachea is actually a main passage of air to the bronchi and then to the lungs, so it is not an accessory structure, it is a main structure needed for the process of respiration.

answer:

it is much easier to find